Bipolar Power Supply Load Current Imbalance And Transformer Core Saturation

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mediatechnology
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Re: Split Power Supply Load Balancing

Post by mediatechnology » Sun Nov 15, 2020 2:19 pm

It has occurred to me that the "full wave" portion of the term "full wave center tapped" may be somewhat of a misnomer.

The FWCT voltage output appears to be full-wave - the same as a four diode "full bridge" - but the current waveform and flux are half-wave in the sense that they occur only on one polarity of peak.
The current and flux peaks are at 2*Fin (resembling FW) but they are still unipolar. (Resembling HW.)
The output is full-wave but the winding and core respond similarly to a half-wave rectifier due to secondary Idc.

Though FWCT and FW-Bridge are both called "full wave" they are entirely different in terms of secondary Idc and how the transformer core responds.
The true FW-Bridge has no Idc other than insignificant levels due to diode mis-match.

Speaking of secondary Idc...

In the FWCT configuration, from an AC perspective, the windings are series-stacked and the end taps are in anti-polarity with respect to the CT.
At DC, the windings appear to be in parallel but only for one polarity. (In the opposite non-conducting polarity they are unconnected.)
It looks like the magnetization from each secondary adds in the core rather than subtracts or "bucks."
This appears to be the case for a transformer with two independent secondaries (four wire) connected to form a common CT.
I'm not sure if magnetization adds or bucks for a three wire secondary with an actual CT.
Not many low-VA universal input transformers are made that way anymore but there may be some CT "filament" transformers with a classic CT used to make a split supply that would behave differently.
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terkio
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Re: Split Power Supply Load Balancing

Post by terkio » Mon Nov 16, 2020 12:04 am

From my simulation: Average currents over an exact period.
WindingA -463.49mA
WindingB 463.49mA
Center Tap -926.98mA

Loads 10 Ohm and 50 Ohm.
1.2301A and 303.13mA
Difference -926.98mA
_________________________
Edit. Now, with balanced loads

WindingA: 0A
WindingB: 0A
Center Tap: 0A
Loads 10 Ohm and 10 Ohm.
1.2301A and 1.2301A
____________________________
Now, with total imbalance.

WindingA: -614.45mA
WindingB: 614.45mA
Center Tap: -1.2289A
Loads 10 Ohm and None.
1.2301A and 0A

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mediatechnology
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Re: Bipolar Power Supply Load Current Imbalance And Transformer Core Saturation

Post by mediatechnology » Wed Dec 30, 2020 10:30 am

"The Effect of DC Current on Power Transformers," Zeimer, 2004.

Big power transformers...
There are a host of adverse effects that can accompany the presence of a direct current in a transformer’s windings. The predominant effect that is witnessed is half cycle saturation. This leads to increased harmonic distortion, increased reactive power losses, overheating and elevated acoustic noise emissions.
"The Effect of DC Current on Power Transformers," Zeimer, 2004. https://proaudiodesignforum.com/images/ ... r_2004.pdf
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CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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